1. Field of the Invention
The present invention relates to an external case for secondary batteries and a secondary battery using the external case, and more particularly, the present invention relates to an external case formed of a metallic material, in which the covering faces for the lower surface and side surfaces of a pouch type core pack are joined such that unfolding due to the resilience of the material is prevented, and so that an increase in the thickness or volume of the battery to be cased is minimized.
2. Description of the Related Art
As there are ongoing increases in the use of portable wireless electronic instruments such as mobile telephones, Personal Digital Assistants (PDAs), digital cameras and the like, and in the weight reduction of electric bicycles, electric vehicles, electric tools and the like, the importance of batteries that are used as the driving power supply for these products is being highlighted, and research is being conducted extensively on the batteries.
In particular, rechargeable lithium secondary batteries have a high energy density per unit weight compared with existing lead acid batteries, nickel-cadmium batteries, nickel-hydrogen batteries, nickel-zinc batteries and the like, due to the lightweight characteristic of lithium atoms, and are also capable of fast charging. Thus, active research is being conducted on further development of the lithium secondary batteries.
Lithium secondary batteries use non-aqueous electrolytes because of the reactivity between lithium and moisture. These electrolytes may be solid polymers containing a lithium salt, or liquids in which a lithium salt is dissociated from an organic solvent.
Furthermore, according to the type of electrolyte, lithium batteries can be classified as lithium metal batteries and lithium ion batteries, both using liquid electrolytes, and lithium ion polymer batteries using polymeric solid electrolytes. The lithium ion polymer batteries can be also classified, according to the type of polymeric solid electrolyte, as all-solid type lithium ion polymer batteries which do not contain an organic liquid electrolyte, and gel type lithium ion polymer batteries which use a gel type polymeric electrolyte containing organic liquid electrolyte.
While the all-solid type lithium ion polymer batteries do not suffer from a problem of the leakage of organic liquid electrolyte, the gel type lithium ion polymer batteries containing an organic liquid electrolyte may have the problem of leakage of the organic liquid electrolyte. However, the liquid electrolyte leakage problem of such gel type lithium ion polymer batteries is ignorable when compared with those lithium ion batteries using a liquid electrolyte. Thus, the liquid electrolyte leakage problem can be prevented by using a multilayer pouch consisting of a sheet of metal foil and one or more polymeric membranes covering this foil. In other words, lithium ion batteries use a firm metal can with considerable strength to prevent leakage of the liquid electrolyte, whereas gel type lithium ion polymer batteries can sufficiently prevent the liquid electrolyte leakage problem simply by means of the multilayer pouch, since the risk of liquid electrolyte leakage is significantly lower in the case of gel type lithium ion polymer batteries than in the case of lithium ion batteries.
Furthermore, using a multilayer pouch is also advantageous in that the weight of the secondary battery can be remarkably reduced, as compared with the case of using a metal can.
The pouch membrane forming the multilayer pouch consists of a sheet of metal foil and polymeric membranes attached to both sides thereof. Aluminum is usually used for the metal foil. The polymeric membranes forming the inner side of the pouch membrane are typically formed of a material which is capable of thermal adhesion, and a polyethylene or polypropylene resin is usually used for the purpose, thereby protecting the metal foil from the electrolyte and preventing short circuits between the anode and the cathode of the secondary battery as well as between the electrode tabs.
In recent years, a configuration conventionally referred to as “hard pack”, in which a protection circuit board is connected to a pouch type bare cell to form a corepack, and the corepack covered with a external case which is then sealed, is widely used. The external case is of a box type having a lower body and an upper cover, with the upper part being open. Such a external case is usually formed of polypropylene resin or the like.
However, since conventional external cases used for the hard pack for pouch type batteries as described above are formed of a synthetic resin, such as polypropylene resin or the like, such hard pack batteries are advantageously lightweight, while being weak in strength. Thus, the external case is easily pressed by an external force, and as a result, the external case itself, or the pouch type battery covered by the external case is easily deformed either in the inward direction or the outward direction. Especially in the case where the external case is deformed in the inward direction toward the battery, the deformed external case exerts a pressure on the bare cell including the pouch, and the protection circuit board or electrode tabs of the pouch type battery may be moved, thereby causing a risk of internal short circuits. Furthermore, the pouch which is relatively weaker in strength may become subject to damage such as tearing, so that the structural safety of the battery in general may be deteriorated.
In addition, in order to address the problems associated with production of external cases from synthetic resins as described above, the external cases may be produced from a metallic material. However, since metallic materials have a feature of exhibiting relatively large resilience or restorability upon bending, external cases made of metal tend to form a surface which is convex in the outward direction from the bent parts, and as a result, the covered pouch type batteries have a problem of acquiring unnecessarily large volumes regardless of the battery capacity. This leads to a large volumetric size of the hard pack batteries regardless of the battery capacity.
Moreover, conventional hardcases made of metal continuously tend to undergo unfolding because of the resilience or restorability of the material itself, during the assembling operation after the external cases are placed to cover the pouch type batteries. Thus, the operations to finish hard pack batteries, such as assembling a hardcase while a pouch type battery is already covered, labeling external cases, and the like, may become very difficult.